An electric or hybrid electric vehicle typically employs electric traction motors connected to drive wheels of the vehicle. The traction motors typically receive electrical energy from an energy bus of the vehicle, in response to which the motors apply a torque to the drive wheels, thereby causing the vehicle to accelerate.
In a “series” hybrid electric vehicle, there are typically two available power sources to supply energy to the traction motors to drive the vehicle: an auxiliary power unit or other energy generating device, and an energy storage system.
The auxiliary power unit typically converts another form of energy into electrical energy which it supplies to the energy bus, but is usually not capable of drawing electrical energy from the energy bus and converting it into another form for storage. For example, the auxiliary power unit often includes an internal combustion engine coupled to an electrical generator, for converting mechanical energy derived from the chemical combustion of gasoline (or other hydrocarbons or other fuels) into electrical energy. More recently, fuel cells for generating electrical energy from other chemical processes such as oxidation of hydrogen for example, have been used as auxiliary power units.
The energy storage system (ESS) typically includes a system capable of both supplying electrical power to the energy bus when needed, and capable of receiving surplus electrical energy from the energy bus and storing such energy for future use. For example, the ESS often includes a battery or a set of batteries, or a capacitor bank. When the auxiliary power unit is not operating (or is not supplying sufficient electrical energy to meet the current demands of the vehicle), the ESS will be called upon to supply stored electrical energy to the energy bus in order to drive the traction motors and other electrical devices of the vehicle. Conversely, when the combustion engine or other auxiliary power unit is operating. It often supplies more electrical energy to the energy bus than is required to operate the traction motors to propel the vehicle and to operate other electrical devices of the vehicle, in which case the ESS may draw a charging current from the energy bus in order to store the surplus energy for future use.
In addition to storing such surplus energy from the auxiliary power unit, the energy storage system may also receive and store surplus electrical energy produced during regenerative braking of the vehicle. In this regard, the traction motors may be used as a regenerative braking system for braking the vehicle, by discontinuing the supply of electrical power to the traction motors while leaving the traction motors fully or partially engaged with the drive wheels. During such regenerative braking, the momentum of the vehicle and resulting forced rotation of the drive wheels causes a corresponding forced rotation of the electric traction motors, which act as generators driven by the drive wheels. Effectively, the electric traction motors serve to decelerate the vehicle by converting its kinetic energy into electrical energy which is supplied back to the energy bus. During regenerative braking, the amount of electrical energy supplied to the energy bus by the regenerative braking system typically significantly exceeds the instantaneous electrical energy needs of the vehicle, because the largest electrical loads, namely, the traction motors themselves, have ceased drawing energy and are now supplying significant amounts of electrical energy back to the energy bus. Therefore, regenerative braking typically produces a significant amount of surplus electrical energy that can be stored by the energy storage system.
Thus, in a series hybrid electric vehicle, there are typically two energy sources capable of supplying surplus electrical energy to charge the energy storage system: the auxiliary power unit, and the traction motors acting as a regenerative braking system.
However, the ability of the energy storage system to safely receive and store energy is typically limited by a number of factors, such as its state of charge, its temperature, its age, and its previous operating conditions, for example. Exceeding the charge acceptance limit of the energy storage system may lead to over-voltage conditions, potentially damaging the energy storage system, and also potentially damaging other electronic components connected to the energy bus.
Therefore, to the extent that the auxiliary power unit and the regenerative braking system may produce surplus electrical energy (i.e., energy in excess of the current electrical needs of the vehicle), if such surplus electrical energy exceeds the amount of energy the energy storage system can safely store, the energy storage system and other electric and/or electronic components of the vehicle may be damaged.
A number of systems have been proposed for monitoring and controlling electrical energy generated by a regenerative braking system and/or an auxiliary power unit, for various purposes. One such system involves detecting a voltage generated by the regenerative braking system, detecting a voltage generated by a generator coupled to an internal combustion engine, and reducing the amount of regenerative braking if the regenerative voltage exceeds the generator's voltage, in order to protect an internal combustion engine against over-speed operation. It is noted that reducing regenerative braking output in favor of the generator output disadvantageously reduces vehicle efficiency.
More significantly, existing systems tend to be reactionary in nature, and often cannot prevent short but damaging voltage spikes from occurring. For example, in a conventional series hybrid electric vehicle, if a user of the vehicle is accelerating at full throttle, the auxiliary power unit will be operating at full power, supplying its maximum amount of electrical energy to the energy bus in order to power the traction motors. If the user then brakes suddenly, the traction motors suddenly switch from operating as a large energy drain to operating as a generator, supplying a large supply of electrical energy to the energy bus, while at the same time, the auxiliary power unit will still initially be operating at or near full power, not having had sufficient time to reduce its output. The resulting surplus electrical energy supplied to the energy bus by the traction motors and the auxiliary power unit typically largely exceeds the charge acceptance of the energy storage system, and the resulting over-voltage condition on the energy bus not only has a detrimental effect on the service life of the energy storage system, but is also potentially damaging to other electronic and/or electrical devices connected to the energy bus. A conventional reactionary system may detect and respond to the over-voltage condition, but typically not until after such potentially damaging voltage spikes have occurred.
Accordingly, there is a need for an improved way of supplying energy in a hybrid electric vehicle.